Mobile wireless terminals and the like use quadrature demodulation. Generally, a local (LO) signal is imparted with a phase difference of π/2 and a frequency conversion is applied using two mixers. Alternatively, if a subharmonic mixer is used, it is necessary that the LO signal be provided with a phase difference of π/4. For example, usually a circuit employing an RC-CR circuit illustrated in FIG. 14 is used as a 90° (π/2) phase shifter for this purpose. Recently, however, it has become common practice to adopt a direct-conversion-type receiving scheme in which a radio-frequency (RF) signal is directly converted to a baseband signal. There are many cases where the local frequency is set to a frequency that is twice the radio frequency and is then frequency-divided by 2 using a Johnson counter comprising flip-flops illustrated in FIG. 15A, whereby a phase difference of π/2 is obtained. FIG. 15B is a diagram illustrating the operating waveforms of the circuit shown in FIG. 15A. Here first and second flip-flops (FF) are driven by a signal 2fLO1 obtained by frequency-multiplying a signal fLO1. A signal fLO1(1) (in-phase signal) is obtained from the Q output of the first flip-flop, and a signal fLO1(Q) (quadrature signal), which has a phase difference of 90° (π/2) with respect to the in-phase signal, is obtained from the /Q output of the second flip-flop.
A receiving scheme referred to as a “low-IF scheme”, which is one type of superheterodyne reception, also is available. This scheme lowers the intermediate frequency (IF) to obtain a frequency band that facilitates integration. The low-IF scheme also uses quadrature demodulation and requires that the local signal be imparted with a phase difference of π/2. In this case, the radio frequency and local frequency differ by the intermediate frequency and therefore this scheme is not that vigorous in relation to the problems of self-mixing and local spurious phenomena. In addition, a phase difference of π/4 imposed upon the local signal required in a subharmonic mixer is attended by particular difficulties in the case of high frequencies.
A phase shifter for obtaining a phase difference of π/4 for this purpose is described in the specification of Patent Document 1 (Japanese Patent Kokai Publication No. JP-A-10-200376). This phase shifter comprises two RC active bandpass filters that employ operational amplifiers in the IF band in the vicinity of 10.7 MHz. Specifically, in a first RC active bandpass filter (BPF) 21, assume that the R and C constants are R1=5 kΩ, R2=10 kΩ and C=1.74 pF, and in a second RC active bandpass BPF 22, assume that the RC constants are R1=5 kΩ, R2=10 kΩ and C=3.10 pF. Even if there are variations in manufacture of the integrated circuit and the absolute values of the resistors R1, R2 and capacitors C shift from their design values, the ratio between the resistors R1, R2 and the ratio between the capacitors C are held substantially constant. Therefore, if the ratio between the CR time constants is taken, it can be expected that this ratio also will be substantially constant.
Accordingly, as illustrated in FIGS. 17A and 17B, which show the amplitude-frequency and phase-frequency characteristics, respectively, it can be expected that the phase difference between the BPFs will be substantially constant even if the center frequency shifts from the design value in the desired BPF.
In actuality, R1 and R2 in the first RC active BPF 21 and second RC active BPF 22 are set to equal values and R1:R2=1:2 holds. Since the ratio between R1 and R2 is an integral value, it is easy to maintain the resistance ratio between the first RC active BPF 21 and the second RC active BPF 22.
With regard to the capacitors C, however, the value is 1.74 pF in the first RC active BPF 21 and 3.10 pF in the second RC active BPF 22. This ratio is 3.10/1.74=1.7816 and is not an integral value.
It is actually very difficult, therefore, to maintain the capacitor ratio at a constant value between the first RC active BPF 21 and second RC active BPF 22. If parasitic capacitance also is taken into consideration, it is essentially impossible to hold constant the value of such a fractional capacitor ratio. It goes without saying that the value of parasitic capacitance accompanying resistor and the value of parasitic capacitance accompanying capacitor have an impact upon the frequency characteristic of an RC active BPF.
The value of parasitic capacitance accompanying resistor and the value of parasitic capacitance accompanying capacitor do not fluctuate that much owing to variations in manufacture, and a value on the order of several tenths of picofarads can be expected. However, if the value of resistance and the value of capacitance each exhibit a manufacture-related variation of ±20% and the sum total of the intended capacitance values (1.74 pF and 3.10 pF, respectively) of the value of parasitic capacitance accompanying resistor and the value of parasitic capacitance accompanying capacitor exhibits a manufacture-related variation of ±20% with respect to the intended capacitance value, then the ratio between the first RC active BPF 21 and second RC active BPF 22 will not be constant.
In the case of high frequency, on the other hand, the combination is changed to a combination of an RC allpass filter and buffer amplifier instead of the combination of RC active BPFs, as illustrated in FIG. 18.
In order to obtain a phase difference of π/4, the R and C values are as follows: R=168Ω, C=1.5 pF in a first RC allpass filter 43, and R=120Ω, C=1.0 pF in a second RC allpass filter 44.
With these values, we have the ratios 168Ω:112Ω=3:2, 1.5 pF/1.0 pF=3:2, all of which are small natural numbers. Even if the sum total of the intended capacitance values (1.5 pF and 1.0 pF, respectively) of the value of parasitic capacitance that accompanies resistor and the value of parasitic capacitance that accompanies capacitor exhibits a manufacture-related variation of ±20% with respect to the intended capacitance value, the ratio between the first RC allpass filter 43 and the second RC allpass filter 44 is set so as to be substantially constant.
Specifically, if we assume that unit resistor and unit capacitor are 56Ω and 0.5 pF, respectively, and letting a, b and c represent parasitic capacitance that accompanies the unit resistor, parasitic capacitance that accompanies the unit capacitor and amount of change per unit capacitor due to variation in manufacture, respectively, the following holds for the first RC allpass filter 43:
                                          R                          total              ⁢                                                          ⁢              1                                =                      56            ⁢                                                  ⁢            Ω            ×            3                                                                    =                          168              ⁢                                                          ⁢              Ω                                ,                                                              C                          total              ⁢                                                          ⁢              1                                =                                    3              ⁢                                                          ⁢              a                        +                          3              ⁢                                                          ⁢              b                        +                          3              ×                              (                                                      0.5                    ⁢                                                                                  ⁢                    pf                                    +                  c                                )                                                                                  =                      3            ×                          (                              a                +                b                +                c                +                                  0.5                  ⁢                                                                          ⁢                  pF                                            )                                          
On the other hand, the following holds for the first RC allpass filter 44:
                                          R                          total              ⁢                                                          ⁢              2                                =                      56            ⁢                                                  ⁢            Ω            ×            2                                                                    =                          112              ⁢                                                          ⁢              Ω                                ,                                                              C                          total              ⁢                                                          ⁢              2                                =                                    2              ⁢                                                          ⁢              a                        +                          2              ⁢                                                          ⁢              b                        +                          2              ×                              (                                                      0.5                    ⁢                                                                                  ⁢                    pf                                    +                  c                                )                                                                                  =                      2            ×                          (                              a                +                b                +                c                +                                  0.5                  ⁢                                                                          ⁢                  pF                                            )                                          
Accordingly, we have the following:Rtotal1×Ctotal1/(Rtotal2×Ctotal2)=3×3/(2×2)=( 3/2)2 Thus, even if there are manufacturing variations and parasitic capacitance, the design is such that the time-constant ratio is held constant between the first RC allpass filter 43 and second RC allpass filter 44. It can be expected that the time-constant ratio will be substantially constant even if the device is implemented in an integrated circuit.
[Patent Document 1] Japanese Patent Application Kokai Publication No. JP-A-10-200376
The conventional circuitry described above has certain problems, described below.
The first problem is large variation. The reason is that implementation is by an open-loop circuit.
The second problem is that it is difficult to provide a function for varying amount of phase electronically or for adjusting it over a very small range.
The reason for the second problem is that the amount of phase is decided by the relative precision of the elements of the RC filter.